1. Field of the Invention
The present invention relates to a process for producing cold rolled steel sheets or strips (hereinafter called as steel strip) having excellent press formability and ageing property using specific steel compositions and specific heat cycles in continuous annealing.
Cold rolled steel strips have been widely used in many applications including automobile bodies, furniture, office instruments, electric appliances and are very closely related with us.
Cold rolled steel strips are nowadays produced in steel making plants with modern equipments, and the most of the steel strip production is press formed with press molds into various complicated shapes for final applications.
Therefore, cold rolled steel strips, which must withstand the severe press forming, are required to have a satisfactory press formability including satisfactory drawability into press molds of cubic structure without fracture, as well as satisfactory press formability, namely the property that the steel strip, when pressed into the mold, hardly surrender to the thickness reduction and the fracture.
Further, cold rolled steel strips are required to have a good ageing property that the above properties will not deteriorate along the elapse of the time after their production, particularly they must be prevented from development of the so-called stretcher strains, or strain patterns which damage the surface quality of the final products.
2. Description of the Prior Art
Conventionally, in order to meet with the above requirements, cold rolled steel strips were produced with application of box annealing, requiring ten or more days for the full treatment. Recently, continuous annealing processes have been more and more widely adapted for the production of cold rolled steel strips, thereby efficient annealing can be performed only in about 10 minutes, as compared with the long time required by the box annealing, and a uniform quality can be obtained through the whole length of the steel strips.
However, the conventional continuous annealing processes have a disadvantage that the workability, particularly press formability of the steel strips obtained is no better than that obtained by the box annealing and the ageing property by the conventional continuous annealing processes is remarkably inferior to that obtained by the box annealing.
Therefore, many efforts have been made by the present inventors for producing cold rolled steel strips satisfying the requirements of both the press formability and the ageing property as mentioned above at low production cost.
Conventionally are known methods in which precious elements, such as titanium and niobium, are added for improving the press formability and the ageing property, but these methods require incorporative use of additional treatments, such as a vacuum degassing treatment, thus necessitating increased production costs. Also, it has conventionally been practiced to raise the hot rolling coiling temperature for the above purposes, but the resultant press formability and ageing property are far inferior to those obtained by the box annealing. Further, most of the cold rolled steel strips produced by continuous annealing have disadvantages that they are susceptible to deterioration of the press formability along the elapse of time while they are left at room temperatures after their production, and they are also susceptible to development of the stretcher strain due to redevelopment of the yield point elongation. These disadvantages are most probably attributable to the fact that the cold rolled steel strips as continuously annealed contain more residual solid solution carbon than the strips obtained by the box annealing.
In order to reduce the solid solution carbon, it has been conventionally practiced to apply an overageing treatment at a temperature from 300.degree. to 450.degree. C., but this is very difficult to reduce effectively the solid solution carbon to the level obtained by the box annealing in a limited time during the continuous annealing process; for example, in only several minutes of overageing treatment. For these reasons, it has been proposed to cool the strips continuously from the annealing temperature to the overageing temperature or to cool the strips to a temperature below the overageing temperature and reheat the strips again to the overageing temperature. In the former case where the cooling is done continuously, the solid solution carbon precipitates at the grain boundaries so that the solid solution carbon is very likely to remain in the strips although a satisfactory drawability and press formability can be obtained. Therefore, the resultant steel strips are inferior in the ageing property, thus failing to provide an ageing property comparable with that obtained by the box annealing.
In the latter case where the reheating is done after the overcooling, the driving force for the carbon precipitation has been given by the overcooling so that the reduction of solid solution carbon can be promoted more rapidly than in the former case, but the precipitated carbide is finely dispersed in the grains so that the resultant press formability tends to deteriorate.
It has been conventionally known that when precipitates, such as AlN, MnS, BN, etc. are present in the steel strips after the soaking, the precipitation of solid solution carbon is promoted by these precipitates which act as the precipitation core so that the reduction of solid solution carbon can be effected rapidly. However, the steel material for the continuous annealing is generally given in the form of hot rolled steel strips as coiled at high temperatures, in which the precipitated particles, such as AlN are coarsened and scarcely dispersed, thus failing to effectively serve as the precipitation core for the solid solution carbon.
Meanwhile, trials of softening the steel by adjustments of chemical compositions, or enlarging the grain growth by a high temperature annealing have been made, but found to be undesirable from the point of the reduction of solid solution carbon, because these trials rather increase the dispersion distance of the solid solution carbon to the grain boundary.
Extensive and detailed studies have been made by the present inventors on chemical compositions of the steels and heat-cycles of the continuous annealing which may have influences on the resultant press formability and ageing property, and it has been found that remarkable improvement of the press formability stretchability can be achieved by lowering phosphorus contents in the steels and that the carbides can be dispersed in a harmless form to the ductility of the steels by controlling the overageing treatment in the continuous annealing step in combination with the lowering of the phosphorus contents, and thus dispersed carbides can be made to serve as the precipitation core for the solid solution carbon, thereby it is possible to produce cold rolled steel strips having press formability and ageing property equal to or better than those obtained by the box annealing.
As a process for producing cold rolled steel strips having excellent press formability and ageing property by continuous annealing, it is known, as disclosed in U.S. Pat. No. 3,920,487, to stepwisely or continuously lowering the strip temperature from the starting temperature, 400.degree. to 500.degree. C., of an overageing treatment to precipitate carbides at grain boundaries at the initial stage of the overageing. This prior art severely avoids the formation of fine carbides in the grain interior so as to prevent deterioration of the press formability stretchability in particular, and has a problem that it is impossible to remarkably lower the yield point elongation simultaneously with remarkable improvement of the press formability stretchability.